When a motor, such as a motor for driving a compressor, is powered by a battery it is normally required that the battery delivers maximum power, or at least very close to maximum power, during start-up of the motor. Such a start-up causes the battery voltage to drop, and it may even cause the battery voltage to become so low that there is a risk that the battery may collapse. It may also be the case that the battery is ‘weak’ and/or the wiring between the battery and the motor is not dimensioned in a correct manner, and in this case the voltage drop may have the effect that the motor is not started at all during the start-up. This is very undesirable.